While there are several good sources, the site that I bookmarked is:
OCA because I can search a feast day/saint by name or by date (plus I happen to like the user interface). Note that the dates are according to the Gregorian calendar.
Irene raised an important point earlier regarding “old style calendar” dates (aka “Julian calendar”), something that wasn’t readily apparent to me initially. This is important to me because I’m dealing with transcripts from an Orthodox church that only recorded dates according to the Julian calendar until the late 1880’s. Thus, I had to do a little bit of date converting (not difficult, but something that I still need to be cognizant of). Perhaps this is not applicable in your case.
The difference for the time period I’m researching (1840 to 1899) was 12 days. Example: December 25, 1849 (Julian) = January 6, 1850 (Gregorian). If you need, a good online date converter is:
Calendar Converter. It's a bit of overkill, but it gets the job done.
Richard